A need exists for a relatively inexpensive device which will indicate when the contamination in the intake air of an air breathing engine, such as an internal combustion engine, becomes excessive. However, since the contamination consists of a distribution of particle sizes of various materials and shapes, precisely defining a single parameter which will correlate with the harmfulness of the contamination in the intake air presents difficulties. One parameter which correlates fairly well with the degree of engine wear caused by the contamination and, accordingly, with the tolerability of the contamination level is the number of particles above a selected minimum size per unit volume in the intake air. Technology exists for counting particles in an air stream. In such systems, the air stream is passed through a light beam and the light scattered from particles in the air stream are detected by photodetectors to generate pulses, which can be counted to provide an indication of the number of particles per unit time. However, the sensors in this technology respond to particles of different sizes, shapes, and materials in different ways. In addition, variations in the location of a particle passing through the light beam causes a variation in the response. As a result, a full analysis of the response of such sensors is quite complex. Moreover, the intake air of an internal combustion engine has a widely varying velocity so that a count of pulses per unit of time will not provide an indication of the number of particles per unit volume in the intake air under normal operating conditions.